The invention relates to zeolite-based adsorbents in the form of agglomerates with a low binder content comprising faujasite zeolite, for their uses in applications in which mass transfer is an important parameter, said adsorbents having a low outer surface area, typically less than or equal to 30 m2·g−1, preferably less than or equal to 20 m2·g−1.
The present invention also relates to a process for preparing said zeolite-based adsorbents, and also to the uses thereof, especially for separating gaseous or liquid mixtures of isomers, more particularly of xylenes and especially for producing very pure para-xylene from an aromatic hydrocarbon feedstock containing isomers containing 8 carbon atoms.
The use of zeolite-based adsorbents comprising at least faujasite (FAU) zeolite of X or Y type and comprising, besides sodium cations, barium, potassium or strontium ions, alone or as mixtures, for selectively adsorbing para- xylene in a mixture of aromatic hydrocarbons, is well known from the prior art.
U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,020 and 3,663,638 show that zeolite-based adsorbents comprising aluminosilicates based on sodium and barium (U.S. Pat. No. 3,960,774) or based on sodium, barium and potassium, are efficient for separating para-xylene present in C8 aromatic fractions (fractions comprising aromatic hydrocarbons containing 8 carbon atoms).
In the patents listed above, the zeolite-based adsorbents are in the form of crystals in powder form or in the form of agglomerates predominantly consisting of zeolite powder and up to 20% by weight of inert binder.
The synthesis of FAU zeolites is usually performed by nucleation and crystallization of silicoaluminate gels. This synthesis leads to crystals (generally in powder form) whose use at the industrial scale is particularly difficult (substantial losses of feedstocks during the manipulations). Agglomerated forms of these crystals are then preferred, in the form of grains, strands and other agglomerates, these said forms possibly being obtained by extrusion, pelleting, atomization and other agglomeration techniques known to those skilled in the art. These agglomerates do not have the drawbacks inherent in pulverulent materials.
Agglomerates, whether they exist in the form of platelets, beads, extrudates or the like, generally consist of zeolite crystals, which constitute the active element (in the sense of adsorption) and of an agglomeration binder. This agglomeration binder is intended to ensure the cohesion of the crystals to each other in the agglomerated structure, but must also give said agglomerates sufficient mechanical strength so as to prevent, or at the very least to minimize, the risks of fractures, splitting or breaks that may arise during their industrial uses during which the agglomerates are subjected to numerous stresses, such as vibrations, high and/or frequent pressure variations, movements and the like.
The preparation of these agglomerates is performed, for example, by pasting zeolite crystals in powder form with a clay paste, in proportions of the order of 80% to 90% by weight of zeolite powder for 20% to 10% by weight of binder, followed by forming into beads, platelets or extrudates, and heat treatment at high temperature for baking of the clay and reactivation of the zeolite, the cation exchange(s), for instance the exchange with barium and optionally with potassium, possibly being performed before and/or after the agglomeration of the pulverulent zeolite with the binder.
Zeolite-based substances are obtained, the particle size of which is a few millimetres, or even of the order of a millimetre, and which, if the choices of the agglomeration binder and the granulation are made according to the rules of the prior art, have a satisfactory set of properties, in particular of porosity, mechanical strength and abrasion resistance. However, the adsorption properties of these agglomerates are obviously reduced relative to the starting active powder due to the presence of agglomeration binder which is inert with respect to adsorption.
Various means have already been proposed for overcoming this drawback of the agglomeration binder being inert as regards the adsorption performance, among which is the transformation of all or at least part of the agglomeration binder into zeolite that is active from the adsorption viewpoint. This operation is now well known to those skilled in the art, for example under the name “zeolitization”. To perform this operation readily, zeolitizable binders are used, usually belonging to the kaolinite family, and preferably precalcined at temperatures generally between 500° C. and 700° C.
Patent FR 2 789 914 describes a process for manufacturing zeolite X agglomerates, with an Si/Al atomic ratio of between 1.15 and 1.5, exchanged with barium and optionally with potassium, by agglomerating zeolite X crystals with a binder, a source of silica and carboxymethylcellulose, followed by zeolitizing the binder by immersing the agglomerate in an alkaline liquor. After exchanging the cations of the zeolite with barium ions (and optionally potassium ions) and activation, the agglomerates thus obtained have identical selectivity results as regards the adsorption of para-xylene with respect to other C8 aromatic molecules and an increase in the para- xylene adsorption capacity, relative to adsorbents prepared from the same amount of zeolite X and binder, but whose binder is not zeolitized. Patent FR 2 789 914 thus teaches that the zeolitization of the binder allows an increase in the para-xylene adsorption capacity, without modifying the adsorption selectivity properties.
Besides high adsorption capacity and good selectivity properties with respect to the species to be separated from the reaction mixture, the adsorbent must have good mass transfer properties so as to ensure a sufficient number of theoretical plates to perform efficient separation of the species in admixture, as indicated by Ruthven in the book entitled Principles of Adsorption and Adsorption Processes, John Wiley & Sons, (1984), pages 326 and 407. Ruthven indicates (ibid., page 243) that, in the case of an agglomerated adsorbent, the global mass transfer depends on the sum of the intra-crystalline and inter-crystalline (between crystals) diffusional resistances.
The intra-crystalline diffusional resistance is proportional to the square of the diameters of the crystals and inversely proportional to the intra-crystalline diffusivity of the molecules to be separated. The inter-crystalline diffusional resistance (also known as the “macropore resistance”) is itself proportional to the square of the diameters of the agglomerates, inversely proportional to the porosity contained in the macropores and mesopores (i.e. the pores with an aperture greater than 2 nm) in the agglomerate, and inversely proportional to the diffusivity of the molecules to be separated in this porosity.
The size of the agglomerates is an important parameter during the use of the adsorbent in industrial application, since it determines the loss of feedstock in the industrial unit and the filling uniformity. The particle size distribution of the agglomerates must thus be narrow, and centred on number-mean diameters typically between 0.40 mm and 0.65 mm so as to avoid excessive losses of feedstock.
The porosity contained in the macropores and mesopores in the agglomerate (the inter-crystalline macroporosity and mesoporosity, respectively) may be increased by using pore-forming agents, for instance corn starch as recommended in document U.S. Pat. No. 8,283,274 for improving the mass transfer. However, this porosity does not participate in the adsorption capacity and consequently the improvement in the macropore mass transfer then takes place to the detriment of the volume-based adsorption capacity. Consequently, this route for improving the macropore mass transfer proves to be very limited.
To estimate the improvement in transfer kinetics, it is possible to use the plate theory described by Ruthven in Principles of Adsorption and Adsorption Processes, ibid., pages 248-250. This approach is based on the representation of a column by a finite number of ideally stirred hypothetical reactors (theoretical stages). The equivalent height of theoretical plates is a direct measurement of the axial dispersion and of the resistance to mass transfer of the system.
For a given zeolite-based structure, a given size of adsorbent and a given operating temperature, the diffusivities are set, and one of the means for improving the mass transfer consists in reducing the diameter of the crystals. A gain in global mass transfer will thus be obtained by reducing the size of the crystals.
A person skilled in the art will thus seek to minimize the diameter of the zeolite crystals as much as possible in order to improve the mass transfer.
Patent CN 1 267 185 thus claims adsorbents containing 90% to 95% of zeolite BaX or BaXK for the separation of para-xylene, in which the zeolite X crystals are between 0.1 μm and 0.4 μm in size, so as to improve the mass transfer performance. Similarly, application US 2009/0326308 describes a process for separating xylene isomers, the performance of which was improved by using adsorbents based on zeolite X crystals less than 0.5 μm in size.
The Applicant has nevertheless observed that the synthesis, filtration, handling and agglomeration of zeolite crystals whose size is less than 0.5 μm involve burdensome, uneconomical processes which are thus difficult to render industrializable.
Furthermore, such adsorbents comprising crystals less than 0.5 μm in size also prove to be more fragile, and it then becomes necessary to increase the content of agglomeration binder in order to reinforce the cohesion of the crystals with each other in the adsorbent. However, increasing the content of agglomeration binder leads to densification of the adsorbents, which causes an increase in the macropore diffusional resistance. Thus, despite reduced intra-crystalline diffusional resistance due to the decrease in the size of the crystals, the increase in macropore diffusional resistance on account of the densification of the adsorbent does not allow an improvement in the overall transfer. Moreover, increasing the binder content does not make it possible to obtain a good adsorption capacity.
It thus appears difficult to obtain adsorbents with all the following properties combined:                the fastest possible mass transfer within the adsorbent, i.e. the lowest possible and ideally virtually zero resistance to mass transfer,        high adsorption selectivity properties for para-xylene with respect to the other C8 aromatic molecules to ensure efficient separation,        the largest possible adsorption capacity (i.e. the largest possible content of zeolite (active crystalline phase in the sense of adsorption)),        an optimum mechanical crushing strength.        